This invention is related to half-bridge direct current-direct current (DCxe2x80x94DC) converter power supplies and more particularly, to a new duty-cycle shifted Pulse Width Modulation (PWM) control scheme for half-bridge DCxe2x80x94DC converters to achieve Zero-Voltage-Switching (ZVS), and to a novel improved half-bridge ZVS DCxe2x80x94DC converter.
With the increasing demand for higher power density in DCxe2x80x94DC conversion with much improved dynamic performance, converter switching frequency continues to increase to reduce the size and cost of passive components. However, with increased switching frequency, a soft switching operation becomes more desirable to reduce the increased switching losses. For isolated DCxe2x80x94DC converters, the phase-shifted full-bridge topology can achieve Zero-Voltage Switching (ZVS) for its four primary-side switches without adding additional switches, and the leakage inductance of transformer and junction capacitance of the Metal-Oxide Semiconductor Field-Effect Transistors (MOSFET) are rationally utilized to achieve soft switching without penalties. See R. Redl, N. O. Sokal, L. Balogh, xe2x80x9cA novel soft-switching full-bridge converter: analysis, design considerations, and experimental results at 1.5 kW, 100 kHzxe2x80x9d, IEEE Power Electronics Specialists Conference Records, 1990, pp.162-172. For high-input-current DCxe2x80x94DC converters, full bridge is preferred owing to lower current stresses, good magnetic core utilization and no leakage inductance related ringing and losses. However, the complexity of the full-bridge is almost highest among the conventional topologies due to its large number of switches, and ZVS switching cannot be achieved at light load. See Praveen K. Jain; Harpreet Soin, xe2x80x9cfull bridge DCxe2x80x94DC Converterxe2x80x9d, 2000 U.S. Pat. No. 6,016,258.
Active-clamp forward topology is another typical example to realize ZVS by utilizing the transformer leakage inductance, magnetizing inductance and MOSFETs junction capacitances. See Cobos, J. A.; Garcia, O.; Uceda, J.; Sebastian, J.; de la Cruz, E., xe2x80x9cComparison of high efficiency low output voltage forward topologiesxe2x80x9d, Power Electronics Specialists Conference, PESC ""94 Record., 25th Annual IEEE, 1994, pp. 887-894. Since the magnetic core is not symmetrically utilized due to leakage inductance, the topology requires a larger transformer.
The conventional half-bridge topology has the same number of switches as active-clamp forward topology but has better transformer utilization. The conventional symmetric PWM half-bridge topology is a popular approach for medium power level applications even though both primary switches operate at a hard switching condition, which limits the switching frequency due to increased switching losses. The complementary (asymmetric) control has been successfully applied to half-bridge DCxe2x80x94DC converters to achieve ZVS for both the high-side switch and low-side switch using the magnetizing current and the output inductor currents. See Pradeep Madhav Bhagwat, xe2x80x9cBias power having a gapped transformer componentxe2x80x9d U.S. Pat. No. 5,663,873, 1997; and P. Imbertson and N. Mohan, xe2x80x9cAsymmetrical duty cycle permits zero switching loss in PWM circuits with no conduction loss penalty,xe2x80x9d IEEE Transaction on Power Electronics, Vol.29, No.1, pp.121-125, 1993. Unfortunately, this scheme leads to asymmetric disadvantages because the two switches operate at different width of duty cycles. See Weiyun Chen; Peng Xu; Lee, F. C., xe2x80x9cThe optimization of asymmetric half bridge converterxe2x80x9d, Applied Power Electronics Conference proceedings, 2001, pp. 703-707. The voltages across the leg""s capacitors are not identical which result in both the current stresses on the primary-side switches and the voltage and current stresses on the secondary-side rectifiers being not identical. This condition requires the use of higher voltage rate rectifiers that in turn degrades the rectification efficiency due to the higher voltage drop and higher reverse recovery losses. Moreover, because the DC voltage gain of the converter is nonlinear, for the same input voltage deviation, a larger duty cycle range is needed, resulting in further degrading in the converter performance. As a result, this asymmetric control is not suited for wide input voltage range applications such as 36V-75V input Telecom modules.
To reduce the duty cycle variation range and voltage stresses applied to rectifiers, an asymmetric winding structure was proposed. See Simon, Fraidlin, Valery I. Meleshin, xe2x80x9cReduced voltage stress asymmetrical DC-to-DC converter using first and second transformers having differing turns ratiosxe2x80x9d U.S. Pat. No. 5,754,413. (1998), which is said to allow the use of lower voltage rate rectifiers to improve the efficiency. Unfortunately, the asymmetric power delivering is still a problem that limits the power delivering efficiency. See Weiyun Chen; Peng Xu; Lee, F. C., xe2x80x9cThe optimization of asymmetric half bridge converterxe2x80x9d, Applied Power Electronics Conference proceedings, 2001, pp. 703-707.
Asymmetric half-bridge is not suitable for wide input voltage range due to asymmetric voltage and current stress. The symmetric control scheme for half-bridge has no asymmetric penalties; but the switches operate at a hard switching condition leading to undesirable switching losses and lower efficiency.
It is an important object of this invention to provide a novel control system for half-bridge DCxe2x80x94DC converters to achieve zero-voltage switching (ZVS).
It is a further object of this invention to provide a soft switching operation for half-bridge DCxe2x80x94DC converters to reduce the switching losses and transformer leakage inductance related losses without asymmetric penalties.
It is a further object of this invention to provide a novel half-bridge DCxe2x80x94DC converter for improved fidelity, higher efficiency, and reliability.
It is a further object of this invention to provide a new soft-switching topology that achieves soft-switching for all the switches of the topology.
According to the invention, there is provided a method of driving a half bridge DCxe2x80x94DC converter comprising means for duty cycle shifted control whereby zero-voltage-switching (ZVS) operation is possible and a new soft-switching DCxe2x80x94DC converter that can achieve soft-switching for all its switches comprising an additional switching network across the input of the isolation transformer of the half bridge.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment which is illustrated schematically in the accompanying drawings.